A new and advantageous extraction procedure was developed for extraction of cerium (IV) from nitric acid with di-n-butylsulfoxide (DBSO) in perchloric acid. The extracted Ce (IV) was determined spectrophotometrically using 0.01 % solution of arsenazo-III in 3 M per-chloric acid. Various parameters such as equilibration time, metal ion concentration, effect of temperature and diverse ions on the extraction of the Ce (IV) were established. U (VI), Th (IV) and Pb (II) interfere whereas only phosphate and fluoride suppress the extraction among anions. The stoichiometric composition of the cerium complex with DBSO was determined by slope analysis and found to be 1:4. The process of extraction was found to be exothermic. Deionized water was the most appropriate solvent for back extraction of cerium. This method is easier and more sensitive than many of the reported procedures.

Keywords: Cerium, extraction, DBSO, perchloric acid, arsenazo-II

1. Introduction

Cerium is the most abundant among rare earth metals and is extracted from monazite, allanite and bastnasite minerals.1 Very pure cerium is used for the production of nuclear power and as a catalyst in nuclear and automobile industries, as an alloy with chromium; in microwave devices, television sets, lasers besides some important uses in environmental pollution assessment, agronomy and biochemistry. In addition to this, many cerium compounds has been found important for biomedical applications.2

In this communication, we report the extraction of Ce (IV) with DBSO in nitric acid using arsenazo-III as chromogenic reagent. The proposed method is highly suitable as almost quantitative extraction of cerium is possible. Higher amounts of cerium can be stripped back to deionized water thus the process is economically more favourable.

2. Experimental

2.2. General Extraction Procedure

Known amount of the cerium sulphate solution was taken in a 100 mL glass beaker and was heated (to near dryness) on an electric hot plate. On cooling, 3.0 mL of the 3.0 M HNO3 was added to it; 2.0 mL of this solution was pipetted out in a screw-capped (18 X 125 mm) culture tube and the remaining 1.0 mL in the 50.0 mL beaker was treated as reference. The aqueous phase was equilibrated with an equal volume of 1.0 M DBSO in chloroform for 5 min and phase separation was achieved with centrifugation. One mL of aqueous solution was pipetted out in another beaker and was heated to near dryness. 3.0 mL of the arsenazo-III (0.01 %) in 3.0 M HClO4 was added on cooling and was mixed thoroughly for colour development. The reagent blank was prepared under identical conditions but without metal ions. The absorbance measurement was made at 658 nm against the reagent blank (Fig. 1). The peak 1 shows the complex. (All presented data are an average of triplicate measurements.)

The distribution ratio (kd) was calculated with the following equation:

In this equation, Ct and Cex indicate the total concentration of the cerium in aqueous phase before and after extraction, respectively.

Accuracy of the extraction method under study has been explored by extracting cerium contents (1000-5000 μg mL-1) from Ce (IV) standard solution (Table 1). A significant decrease in percentage deviation was observed while increasing cerium concentration. It was noted that loading capacity of DBSO for cerium is quite high to extract the metal ions present in the concentration range studied but decrease in percentage deviation of cerium at its high concentration was probably due to fluctuations caused by the dilution at spectrophotometric step. Another possible reason for the decreasing percentage deviation might be the higher metal ion concentration as nitric acid competition for extractant decreases and maximum extraction of cerium occurs which resulted an increase in the expected error. Therefore, we noted that in this extraction procedure, lower concentration of metal ions gives better results.

3. Results and Discussion

3.1.Effect of Nitric Acid Concentration

Dependency of percentage extraction of cerium with different concentrations of nitric acid (using 1.0 M DBSO in chloroform) was studied (5 min shaking time). Equal volume (2.0 mL of each) of organic and aqueous phases was equilibrated. An abrupt increase in the percentage extraction of cerium with increasing concentration of nitric acid reaching a maximum value (76 % for 3.0 M HNO3) was observed (Fig. 2). Further increase of the nitric acid concentration caused a decrease in the percentage extraction of cerium in the range of 0.5 to 7.5 M. At lower acid strength, percentage extraction was higher due to mild competition between cerium and acid for complexation with DBSO. This might be due to the fact that increasing acid concentration decreases free DBSO concentration thus leading to a stronger competition between metal (cerium) and acid (HNO3) for complexation with the extractant. Coordination of DBSO with HNO3 results into (nDBSO.mHNO3) adduct. Thus after reaching a maximum (with 3 M nitric acid), percentage extraction decreases with increasing acid concentration.

3.2.Effect of Equilibration Period

The shaking time duration affects the extraction of cerium (IV) using 1.0 M DBSO and has been studied while keeping all the parameters constant under previously optimized experimental conditions (Fig. 3). The results are indicative of a sharp enhancement in percentage extraction by increasing shaking period (for 5 min) and then a decrease with increasing shaking time. This can be explained on the basis of the fact that equilibration between cerium and DBSO in this system reaches equilibrium saturation with a high rate constant that no further increase in extraction was observed (after 5 min shaking time). Therefore, it appears that equilibrium for Ce(NO3)4.nDBSO is faster than that of Ce(NO3)4.nDBSO.mHNO3. After 5 min more of the latter forms, resulting in less efficient extraction of the metal. In conclusion, disparity of shaking time indicated that a single extraction with DBSO (1.0 M) in chloroform for a maximum of 5 minfrom HNO3 (3.0 M) solution is appropriate for cerium extraction (up to 75 %). The concentration range of Ce was 10.77 X 10-3 to 3.56 X 10-3 mol dm-3.

3.3.Effect of Extractant Concentration

The impact of extractant concentration was explored by changing the concentration from 0.3 to 2.0 M DBSO using chloroform as diluent. All other parameters were kept constant under previously optimized experimental conditions. A curve was constructed between different molarities of DBSO solution versus extraction of cerium (Fig. 4). Maximum percentage extraction was with 1.0 M DBSO and then a decrease was noted. Two factors are potentially responsible for the decrease in the percentage extraction of cerium (with increased concentration of extractant). Firstly, due to extraction of more nitric acid by higher concentration of DBSO solution, aqueous acid concentration decreases resulting into a lower Kd value of cerium. Secondly, the volume of the organic phase increases due to more extraction of acid by higher DBSO concentration thus decreasing cerium concentration in the organic phase. Therefore, 1.0 M DBSO (solution in chloroform) was selected as maximum concentration of the extractant for quantitative cerium (IV) extraction. The concentration range of Ce was 10.77 x 10-3 to 3.56 x 10-3 mol dm-3.

Since DBSO makes coordinate covalent bonds with Ce(NO3)4, it is therefore to vary the concentration to fully understand its role in the extraction process.

3.5. Effect of a Salting-out Agent

Varying concentrations of aluminum nitrate Al(NO3)3.9H2O were used during extraction of cerium keeping other parameters constant (Table 3). On adding nitrate (from aluminum nitrate), the extraction efficiency was increased probably due to participation in the metal solvation process. However, with low acid concentration, the nitrates increase extraction by common ion and salting-out effects.

3.6. Effect of Anions

The effect of various anions such as PO4-3,F-,SO4-2,CN-,Cl-and S2O3-2 on the extraction of cerium was studied by mixing the sodium or potassium salts (Table 4). Anions such as PO4-3 and F-have reduced the percentage extraction while sulphate, cyanide, chloride and thiosulphate have comparatively less effect. The serious interference of PO4-3 and F- was due to complex formation with Ce in aqueous solution. The formation constants of these ions with cerium are higher than with nitrate. These ions act as masking agent for cerium extraction. Therefore, interfering anions must be separated from the aqueous phase prior to extraction.

3.7. Effect of Cations

The effect of various cations such as uranium, thorium, zinc, lead, aluminum, and sodium on the extraction of cerium was studied (Table 5). The data indicated that lead, thorium and uranium have reduced the normal percentage extraction of cerium while cations such as zinc and sodium have comparatively less effect. An increase in percentage extraction of cerium was noted for Al (III).

3.8.Effect of Temperature

The temperature effect (25 to 50 °C) on the extraction process was explored. The percentage extraction of Ce (IV) has an inverse relation with increasing temperature. Thus, an increase in temperature does not favour the extraction of cerium because of competition by the other extractable species (Fig. 5). Hence, a lower temperature is favourable for Ce (IV) extraction process used herein.

3.9.Stoichiometric Composition of the Extracted Complex

Stoichiometric composition of the cerium DBSO complex was investigated using a slope analysis method (using 3.0 M HNO3 as aqueous phase). Different concentrations of DBSO solution (in chloroform) were used (Figs 6, 7). The following theoretical equation was used to establish the extraction mechanism.

where n is the number of DBSO molecules.

By taking log on both sides of equation:

By rearranging

Equation 5.1 represents a straight line equation giving the slope equal to n and the intercept equal to logKeq + 4 log(NO3)~. By using the equation (1), a log-log plot of the distribution ratio versus DBSO concentration at constant concentrations of metal ion and nitric acid and is shown in Fig. 7. Slope (2) suggested that two molecules of DBSO are attached with Ce(NO3)4.2DBSO.

Cerium preferably coordinates with coordination number 6 or 8 but higher coordination numbers are also possible.44 If it is assumed that nitrate ions in this complex are linked to cerium in a mono-dentate mode, then presence of four mono-dentate nitrate groups and the additional two mono-dentate sulfoxide molecules with cerium in the resulting complex give a coordination number of 6.

3.10. Stripping of Extracted Cerium

Aqueous solutions of nitric acid (0.1 M), ammonium hydroxide (0.1 M) and deionized water were used for cerium stripping from the organic phase (DBSO in chloroform, and ammonium hydroxide solution (0.1 M) was found to be the most effective (Table 6). Recovery of cerium with water and 0.1 M HNO3 was found to be 89 % and 75 %, respectively. Deionized water was finally chosen as stripping agent based upon the economic consideration and extraction effectivity of cerium from organic phase.

3.11. Recycling of the Extracting Agent

The recycling ability of DBSO (in chloroform) for the extraction of cerium was studied and the process was repeated four times and results are shown in Table 7. Extracting potential of the organic solvent has been checked out and found to be the same after back extraction without any deterioration.

4. Conclusion

This method for the cerium extraction is significantly simple, sensitive, selective and reproducible for rapid determination of cerium (IV). Almost quantitative extraction of cerium (IV) from 3.0 M nitric acid solution could be achieved with 1.0 M DBSO solution in chloroform. Higher amount of cerium (89 %) can be stripped back into the deionized water.